In this description we use the term gearwheel to refer to a cogged, rotating machine part. Two or more meshing gearwheels constitute a gear stage. The term gear as such refers in this description to a mechanical system having a first shaft and a second shaft, between which one or more gear stages provide speed and torque conversions and/or a change in a direction of a rotation axis. A gear unit comprises a gear proper and may comprise auxiliary augmenting systems, such as instrumentation, control, and lubrication arrangements.
A lubrication system of a gear unit comprises typically a lubrication pump that is arranged to circulate lubrication fluid, such as oil, through a gear stage or gear stages of the gear unit and through bearings of the gear unit. As the viscosity of the lubrication fluid, especially in case of lubrication oils, is usually strongly dependent on temperature, the lubrication pump has to be designed and operated in way that the lubrication pump is not damaged even if the lubrication fluid is cold and thereby its viscosity is relatively high.
Due to this high viscosity of the cold lubrication fluid, especially when starting up a machinery comprising the gear unit, the viscosity of the lubrication fluid often need to be lowered by warming up the lubrication oil. This warming up of the lubrication fluid is usually achieved by equipping the lubrication system of the gear unit with pre-heaters.
The use of the pre-heaters and robust design specifications for the lubrication fluid pump, among others, raises the complexity and cost of a gear unit.
Another type of lubrication method used in enclosed gear units is splash lubrication. In splash lubrication, the gearwheel tooth dip into a tray of lubricant and transfer the lubricant to the meshing gearwheels as the gearwheels rotate.
Publication DE 32 31 016 discloses a solution where circulation lubrication and splash lubrication is used in the same gear unit. In this solution the change from circulation lubrication to splash lubrication takes place by means of a control device when the current supply to the gear unit is cut off. This way the gearwheels of the gear unit remains lubricated even during electrical power failure.
Publication GB 2 201 200 discloses a method of lubricating gear means for a wind energy installation that comprises the steps of maintaining operational lubrication by an oil-circulating system with injection lubrication, with the oil being cooled at the same time. In this solution the normal height of the oil level in the gear means casing is increased to a height necessary for splash lubrication when the rotational speed of the gear means falls below a settable value.